RF power combiners, with odd number of input ports, are generally of the "Wilkinson" type as initially described in the article "An N-Way Hybrid Power Divider", IRE Transactions On Microwave Theory and Techniques, Vol. MTT-8, pp. 116-118, Jan. 1960 as well as in U.S. Pat. No. 3,091,743 dated May 28, 1963. Such combiners obtain input port-to-port isolation for each port by feeding each of the other ports with the signal applied to any one port through resistors with a 180 degree phase shifted voltage, with one-quarter wavelength transmission lines providing the 180 degree phase shift required for cancellation. At the frequencies of interest, e.g., 30 MHz, the physical length of the one-quarter wavelength transmission lines becomes impractical for many applications.
As an alternative, Wilkinson type combiners have been built using ferrite transformers instead of the one-quarter wavelength transmission lines. Such a combiner is described in the U.S. Pat. No. 3,428,920 dated 1969. Excellent port-to-port isolation can be obtained at very low power (less than one watt) for bandwidths of 30 MHz or less. However, the phase shift and amplitude must be carefully controlled as a function of the frequency of the signals combined when a wide bandwidth is desired.
The ferrite core technique has been described in the Motorola RF Device Data Manual, Vol. 2, pp. 7-102 to 7-105, with short transmission lines loaded with ferrite to minimize the additional phase shift in low power applications, e.g., a few hundred watts. However, when the ferrite cored transformers are required to handle kilowatts of power, the increase in size results in excessive phase delays with significant deterioration in port-to-port isolation.
It is accordingly an object of the present invention to obviate many of the problems of the known prior art and to provide a novel power combiner and method with excellent port-to-port isolation.
A further object of the present invention is to provide a novel RF power combiner and method which maintains port-to-port isolation over a significant bandwidth e.g., 30 MHz.
It is another object of the present invention to provide a novel RF combiner and method capable of handling five kilowatts or more of power.
It is yet another object of the present invention to provide a novel combiner and method of maintaining port-to-port isolation between two ports of unequal impedance.
It is yet a further object of the present invention to provide a novel RF power combiner and method which maintains port-to-port isolation between three ports of equal impedance.
It is still a further object of the present invention to provide a novel RF combiner and method which significantly reduces the number of transformers required.
It is yet still a further object of the present invention to provide a novel RF transmission system in which the output signals from three amplifiers (driven by one or more exciters) with equal output impedance are selectively combined for application to a single antenna with the same input impedance.